236 LECTURE XII. 



found in various parts of many plants : its derivation from tannic acid 

 has been mentioned above. 



The resins are substances which are nearly always to be found in 

 plants and in all parts of them. Three kinds of resins may be distin- 

 guished, (a) True -resins ; () Balsams, mixture of resins and ethereal oils, 

 together with aromatic acids ; (c) Gum-resins, mixture of resins, gums, 

 and ethereal oils. 



Ordinary resin (colophony) obtained from the Conifers appears to be 

 a mixture of resinous acids, the most important of these being Abietic 

 anhydride, C4 4 H 62 O4 (Maly). Resin is produced in cells, and it is ex- 

 creted into intercellular spaces termed resin-passages. According to the 

 researches of Wiesner, of Franchimont, and of Hlasiwetz, it appears that 

 the first step towards the production of resin is the formation of a gluco- 

 side, probably tannin ; the tannin then undergoes decomposition, and 

 from the tannic acid resin is derived. Franchimont, especially, draws 

 attention to the relation of the cells containing tannin to the resin-pas- 

 sages in Conifers. He is of opinion that the decomposition of the glu- 

 coside is attended by the formation of oxalic acid. He also points out 

 that the secreting cells lining the resin-ducts do not themselves contain 

 resin, but that they excrete into the ducts a substance which becomes 

 converted by oxidation into resin. It seems probable that from the 

 tannic acid an ethereal oil (terpene) is formed in the cells, that this is 

 excreted into the ducts, and that it then undergoes partial oxidation into 

 resin, the turpentine which the ducts contain being a solution of resin 

 in an ethereal oil. Hlasiwetz represents the process of oxidation of 

 terpene into resin thus, 



2C 10 H 16 + 3 = C 20 H 30 2 + H 2 0. 



The group of ethereal oils includes a great number of substances of 

 very various chemical composition, which are present to a greater or 

 less extent in all parts of Flowering Plants: it is to the presence of 

 volatile ethereal oils that the odours of plants are due. These oils may 

 be classified into those which do and those which do not contain 

 oxygen. 



a. Ethereal oils containing oxygen ; the following are the more 

 common : 



Aldehydes : oil of cinnamon, C 9 H 8 O ; oil of bitter almonds C 7 H 6 O. 



Ketones : oil of rue, methylnonyl ketone, CuH a O : camphor, C 10 H 16 O 

 is probably a ketone. 



Acids : oil of cloves or eugenol, C 10 H 12 O 2 ; coumarin, C 9 H 6 O 2 , to which 

 the odour of the Tonka Bean, and of new-mown hay is due, is 

 probably the anhydride of coumaric acid: eugenol is also re- 

 garded as being an ether-alcohol. 



